Industrial applications of liquid hydrocarbons and particularly, liquified olefinic hydrocarbons, has become more increasingly specialized. The technology as presently developed utilizes highly efficient catalysts to convert these liquified hydrocarbon feedstocks into final product such as polymers. However, these highly efficient catalysts are very sensitive to contaminants, particularly sulfur contaminants, found in these hydrocarbon feedstocks.
In addition to the well known sulfur compounds such as hydrogen sulfide and mercaptans, the hydrocarbon feedstocks normally contain a small quantity of carbonyl sulfide (COS). Usually COS is present to the extent of only several hundred parts per million (ppm) by weight. However, even this small amount is normally beyond the allowable limits of an acceptable product. Since carbonyl sulfide is almost always formed when carbon, oxygen, and sulfur or their compounds, such as carbon monoxide, carbon disulfide and the like, are brought together at high temperatures, this compound is most frequently found in the hydrocarbon feedstocks resulting from thermal and/or catalytic cracking operations, although, in some cases, it has been found in virgin petroleum fractions.
To some extent, carbonyl sulfide is not as reactive as its companion in hydrocarbons, hydrogen sulfide. According to Kirk-Othmer's Encyclopedia of Chemical Technology, Vol. 13, pages 384-386, 1954 edition, carbonyl sulfide reacts slowly with the aqueous alkali-metal hydroxides and is only slowly hydrolyzed to carbon dioxide and hydrogen sulfide. This relatively unreactive characteristic of carbonyl sulfide makes it extremely difficult to remove from petroleum streams by conventional desulfurization techniques.
The presence of COS, even at very low concentrations, oftentimes renders olefins valueless for many purposes. For example, high purity olefins are required for the satisfactory production of many polymeric products, especially those useful as plastics, including polymers of ethylene, propylene, and the like. As a result, there has been a real need to improve techniques for removing COS from hydrocarbons, especially those used for polymer production.
Some of the known methods for removing carbon oxysulfide (COS) from hydrocarbon streams include the following. In British Patent Specification No. 1,142,339, published Feb. 5, 1969, the inventors teach a process for the removal of COS from gas mixtures in which unsaturated compounds such as propyne and propadiene are present, comprising passing said mixtures in liquid phase at atmospheric or superatmospheric pressures over a substance which contains one or more of the oxides of cadmium, zinc, nickel or cobalt supported on a carrier. It is stated that this process reduces the COS concentration to less than one (1) ppm.
U.S. Pat. No. 4,290,879 to Woodall et al, teaches the removal of carbonyl sulfide from propane and other similar liquified petroleum gas products by mixing liquid methanol with the untreated liquified gas and subsequently contacting the liquid mixture with solid potassium hydroxide. The COS concentration is reduced to less than one (1) ppm by volume.
U.S. Pat. No. 3,315,003 to Khelghatian, teaches that carbonyl sulfide can be effectively removed from normally gaseous hydrocarbons by first liquifying the hydrocarbons and then contacting them with soda-lime. The effluent gas must subsequently be dried to remove the moisture therefrom.
U.S. Pat. No. 3,284,531 to Shaw et al, teaches that COS can be removed by passing a fluid hydrocarbon through a bed of an anhydrous, weakly basic, anion exchange resin.
U.S. Pat. No. 3,282,831 to Hamm, discloses a method for removing COS from a hydrocarbon stream by utilizing an anionic exchange resin which is in the hydroxyl cycle and which is not fully hydrated.
The problems in purifying propylene and the like olefins are singularly complicated by the nearly identical boiling points of propylene and COS which makes COS removal by fractionation unsuitable. As a result, the levels of COS impurity in propylene stocks are often times intolerably high.
Still other disadvantages are encountered in the heretofore known procedures for the removal of COS from hydrocarbons, particularly those to be used for olefin polymerization. For example, some of the established methods introduce water or other contaminants into the hydrocarbon stream, all of which must be removed by additional processing in order to place the hydrocarbon in suitable condition for use. Any such additional processing, as well as any requirement to employ elevated temperatures adds materially and undesirably to the cost of the operation.
None of the above methods can reduce the COS content to less than fifty (50) parts per billion (ppb) by weight. Accordingly, it can be seen that there is a need for a process to reduce the COS concentration in a hydrocarbon stream to 50 ppb by weight or lower.